And here's a perfect situation to take advantage of Sub::Lexical's ability to emulate nested subs ...

use Sub::Lexical;

sub find_key_dupes {
  my $binary_tree=shift;
  my $key=shift;
  my @dupes;

  my sub get_dupes($) {
    my $node=shift;
    return unless $node;
    if ($node->{key} lt $key) {
      get_dupes($node->{right});
    } else {
      push @dupes,$node if $node->{key} eq $key;
      get_dupes($node->{left});
    }
  }

  get_dupes($binary_tree);
  return \@dupes;
}
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Thanks demerphq, I'll be using this as valid example code in future. <simpsons voice="Mr. Burns">ehhhxcellent</simpsons>.
HTH

_________
broquaint